A semiconductor device is an active element-including electronic device using electrical characteristics of a semiconductor. Such a semiconductor device has been widely used in an audio device, a communication device, a computer, an electric appliance and the like. Among these, a TFT array substrate has been known as a semiconductor device including a thin film transistor (hereinafter, also referred to as a “TFT”) on a substrate. Such a TFT array substrate has been used as a component of an active matrix driving liquid crystal display device and the like.
A system-on-glass technology in which a pixel circuit part and a driving circuit part are formed on the same substrate using polysilicon (hereinafter, also referred to as “p-Si”) as a semiconductor material for a TFT has been recently used in the active matrix driving liquid crystal display device. Due to this technology, a TFT in the pixel circuit part (hereinafter, also referred to as a “pixel switching TFT”) and a TFT in the driving circuit part (hereinafter, also referred to as a “driving circuit TFT) are integrally formed, which permits downsizing, a reduction in electric power consumption, and high reliability of the liquid crystal display device. However, different characteristics are needed between the pixel switching TFT and the driving circuit TFT. That is, for the pixel switching TFT, a low off-state current is needed to suppress a reduction in contrast ratio or uneven image qualities inside a panel, but for the driving circuit TFT, a reduction in threshold voltage (Vth), a reduction in variation of Vth, and a high on-state current are needed in order for the driving circuit to operate at a high speed. Accordingly, in order to satisfy these needed characteristics, the pixel switching TFT and the driving circuit TFT need to have different structures.
For such a need, a method of forming a gate insulating film of the pixel switching TFT to have a thickness larger than that of a gate insulating film of the driving circuit TFT has been known, for example. For example, a thin film semiconductor device in which a TFT in a matrix circuit part, including a gate insulating film having a two-layer structure between a semiconductor layer and a gate electrode (a pixel switching TFT) and a TFT in a peripheral circuit part, including a gate insulating film having a single-layer structure (a driving circuit TFT) are formed on the same substrate is disclosed (for example, refer to Patent Document 1). In this thin film semiconductor device, the gate insulating film of the TFT in the peripheral circuit part has a silicon oxide or silicon nitride single-layer structure. The gate insulating film of the TFT in the matrix circuit part includes a lower part having the same structure as in the gate insulating film of the TFT in the peripheral circuit part. Also, the gate insulating film of the TFT in the matrix circuit part includes an upper part having a silicon oxide or silicon nitride single-layer structure. However, according to this configuration, in the TFT in the matrix circuit part, it is impossible to continuously form the two layers, i.e., the upper part and the lower part of the gate insulating film, and on the interface between the upper part and the lower part, many traps exist. Therefore, if the lower part of the gate insulating film has a silicon oxide single-layer structure, the reliability of the TFT in the matrix circuit part is reduced. In such a point, this thin film semiconductor device has room for improvement. Further, the upper part of the gate insulating film is formed by a sputtering method or a plasma CVD method. Therefore, if the lower part of the gate insulating film has a silicon oxide single-layer structure, the lower part is plasma-damaged in the step of forming the upper part of the insulating film, which further reduces the reliability of the TFT in the matrix circuit part. In such a point, this thin film semiconductor device has room for improvement. In addition, if, in the TFT in the matrix circuit part, the lower part of the gate insulating film is a silicon nitride layer, it is impossible to form an excellent interface between the gate insulating film and the semiconductor layer (polysilicon layer), which fails to obtain excellent TFT characteristics. In such a point, this thin film semiconductor device has room for improvement.
Further, a production method of a semiconductor device, in which the first and second field-effect transistors including gate insulating films with different thicknesses are formed by forming an insulating film constituting a gate insulating film, and then selectively removing a specific region of the insulating film by etching, is disclosed (for example, refer to Patent Document 2). However, according to this production method, the insulating film to be etched has a single-layer structure. Therefore, if the above-mentioned etching is dry etching, the part formed after the etching is plasma-damaged, which might possibly reduce the reliability. In such a point, the method has room for improvement.
Accordingly, in the conventional production method, if the pixel switching TFT and the driving circuit TFT are formed on the same substrate, in either one TFT, the gate insulating film is deteriorated or damaged, and thereby the reliability is reduced. Therefore, mass production of such a semiconductor device has not been achieved yet.    [Patent Document 1]    Japanese Kokai Publication No. Hei-05-335573    [Patent Document 2]    Japanese Kokai Publication No. 2005-72461